The present invention concerns fluid mixtures of alkali metal silicates, water and a gelling agent, which stiffen within a predetermined length of time to form gels containing silica. An important field of application is the stabilization of the soil in building construction.
The separate or combined use of aqueous solutions of alkali silicate and gelling agents for soil stabilization has long been known. The most commonly used gelling agents are those which can be mixed with the silicate solution without immediately forming a gel, and which produce the gelling action after a time sufficient for the preparation and application of the mixture. By the injection of these liquid mixtures into the ground, permeable soils can be sealed and strengthened.
The properties and the adaptation of the gels are controlled by the nature and amount both of the alakli silicate and of the gelling agent.
The gels must have a low and slow syneresis to produce an effective sealing of soil. The term "syneresis" refers to the shrinkage of the gels, accompanied by the segregation of an aqueous phase (syneresis water).
The use of water glass solutions with sodium aluminate as gelling agent, which is known as the monosol process, satisfies these requirements in the low range of water glass concentrations. These gels have a good sealing action, but virtually no inherent strength. A substantial improvement of the gel strength by increasing the water glass concentration and hence the silica content of the gels is not possible, because if mixtures are prepared with a silica content of more than about 110 grams per liter, the danger exists of an immediate gelling or precipitation upon the addition of the sodium aluminate. Consequently, only gels having a maximum silica content of about 120 grams per liter can be produced in the soil, and their strengthening effect is very poor.
Other gelling agents, which are frequently organic, are used in strengthening soils with mixtures based on alkali silicates. The organic agents, mainly esters as well as a number of amides and aldehydes, can also be admixed with high-content water glass solutions without the danger of an immediate reaction.
For example, it is known from French Patent Nos. 1,164,835 and 1,502,645 to use alkyl esters of low fatty acids in mixtures with alkali silicate solutions for the stabilization of soil. According to German Patent Nos. 1,567,776 and 2,242,713, the alkyl esters of dicarboxylic acids, such as succinic acid, glutaric acid and adipic acid, are suitable as gelling agents. A disadvantage of most of the alkyl esters, including ethyl acetate, is their poor solubility in water glass solutions, which can result in separation of unreacted esters as an upper layer. Irregular gelling and contamination of ground water may be the result.
To avoid these disadvantages, a number of water-soluble esters are recommended as gelling agents. In French Patent No. 2,175,481, glycerine acetate mixtures, and in British Patent No. 1,109,140, cyclic esters of the lactone or alkylene carbonate type, are named as suitable gelling agents for alkali silicate solutions.
The high strength and hence also the strengthening action of the gels produced by means of organic gelling agents is based mainly on their high silica content. This is to be attributed not only to the higher silicate concentrations of these mixtures but also to the great syneresis of the gels. With the shrinkage of the gels, which can amount to about 50 to 70% in the case of medium silicate concentrations (approx. 45 to 70% by volume of technical water glass solution of a density from about 1.25 to about 1.45, corresponding to 30 to 45.degree. Baume), their percentage content of silica increases by a similar amount. While the strength of the gels increases with syneresis, their sealing action decreases, since the pore volume of the treated soil is still only partially filled with the gel. Upon an increase in permeability to moisture, or upon a sudden invasion of water, considerable amounts of organic hydrolysis products and an unchanged residue of the gelling agent can be carried along with the syneresis water into the ground water and this can result in an unacceptably great pollution.
In practice it is often necessary both to seal and to strengthen a site. The formation of a gel to fulfill both requirements requires a suppression of the syneresis that might result in permeability. The possibilities for this purpose are: (1) to increase the silicate content of the gel forming mixture to more than 75%, for example, of the volume of the technical water glass solution, thereby suppressing the segregation of syneresis water; and (2) to bind or entrap the syneresis water by the addition of structurizing agents, such as cement suspensions for example. In both these cases, the above-named organic substances have been used as gelling agents, and in the second case formamide, especially, has been used; the use of formamide, however, is objectionable on account of its toxic (teratogenic) effect. For the treatment of finely granular soils, however, neither of these two possibilities can be contemplated, since the penetrating ability of the mixtures is greatly reduced, in the first place by the high viscosity, and in the second case by the solid content. In practice, therefore, such finely granular soils are first treated with a sealing mixture, and then the strengthening mixture is injected. Such a procedure is very time-consuming and is virtually twice as difficult and time-consuming as a onestep procedure.
It is therefore the object of the present invention to develop mixtures on an alkali silicate basis which will be suitable for soil stabilization and which not only will assure a reliable sealing action but will also strengthen soils and be suitable for the treatment of finely granular soils.